1. Field of the Invention
The present invention relates to an optical integrated circuit and a method of manufacturing the same. More particularly it relates to a PLC based optical integrated circuit including a light path turning micro-mirror inside the optical waveguide and a method of the same.
2. Description of the Related Art
The transmission capacity of optical communication system and optical interconnection apparatus has been continuously increased to meet the rising demand of wired/wireless communication traffic. A transmission distance of an optical cable decreases as data rate increases same as a copper cable. Accordingly new transmission technologies such as a multi lane transmission techniques and a higher order modulation technique in combination with a coherent detection technique has been developed.
Among the above technologies, the multi lane transmission technique is divided into two groups; (1) increasing the number of physical transmission paths and, (2) a wavelength division multiplexing (WDM) technology which transmits optical signal having a plurality of different wavelengths through a single optical fiber.
A transmission method using a multiple optical fiber is economical when the transmission distance is very short, for example, equal to or less than several hundred meters. However, when the transmission distance is longer than that the WDM technique is more economical.
A typical WDM system includes optical transmitters and optical receivers which convert electrical signal to light signal and light signal to electrical signal, respectively. Optical transmitters include a plurality of light sources each of them generating a different wavelength of lights and a multiplexer (MUX) which combines the lights having different wavelengths and sends its output to optical fibers. Optical receivers include a demultiplexer (DMUX) which spatially separates the muxed light signal into multiple light channels depending on the pre-determined wavelengths and a plurality of photodetectors receiving the light signals from each separated light channels and converting the light signal to electrical signal.
An arrayed waveguide grating (AWG) is one of the most popular MUX and DMUX device with a MUX/DMUX based on thin film filter technology. An AWG is also one of a key component consisting a planar lightwave circuit (PLC). AWGs can be made with a variety of materials including polymer, silicon oxide (SiOx), or silica on silicon or glass substrate. Among them, silica on silicon or glass has been the most widely used for the manufacturing of AWGs because it has lowest insertion loss. Wherein, the PLC means a lightwave circuit having optical waveguides for active or passive components in which a propagation path of a lightwave signal is substantially parallel to the surface of the lightwave circuit. The PLC is an essential element of a hybrid and monolithic optical integrated circuits.
In optical telecommunication systems, laser diodes and photodetectors are the most representative electrical-optical (E-O) and optical-electrical (O-E) conversion devices, respectively. Among photodetectors, surface illuminated type photodetectors which have a light detection active region on the surface is more common than alternative waveguide type photodetectors.
A vertical cavity surface emitting laser (VCSEL) is a surface emitting type light source emitting light from the front surface. Conventionally, there are two different types of optical coupling scheme between a PLC and surface illuminated type photodetectors or surface emitting type lasers. One is a parallel optical coupling scheme which maintains a light path parallel to a substrate and the other one is an edge optical coupling scheme which transforms a light path perpendicularly to a substrate.
The optical coupling scheme between photodetectors and a PLC, and between VCSELs and a PLC have the same basic principle except they have opposite in/out paths to each other. And thus, hereinafter, light coupling structures between a PLC and photodetectors will be described in detail.
In parallel optical coupling scheme between a DMUX AWG and surface illuminated type photodetectors, photodetectors are mounted perpendicularly in front of an end face of optical waveguides in order to receive lights from output waveguides of DMUX AWG. In general, the photodetectors and VCSELs have very thin thickness of about 0.1 mm to 0.2 mm, therefore it is difficult to mount the chip itself on a substrate. Therefore, photodetectors are usually attached on a third submount having an appropriate area and thickness and then the submount is vertically mounted in front of the optical waveguides end faces.
Those parallel optical coupling scheme between optical waveguides and active optical elements has several disadvantages: has a large volume, the end faces of the optical waveguides have to be polished, produces a large electrical signal attenuation caused by a long electrical connection path between the photodetector and an electrical signal processing element, and a complexity of alignment process between the photodetector and the optical waveguide.
In a vertical or edge optical coupling scheme, the light propagation path of output light from optical waveguide of a PLC is changed vertically upwards or downwards of waveguide substrate and then photodetectors are directly attached on the surface of PLC. The above-described vertical optical coupling scheme can avoid the use of submount. In addition, automated or semi-automated surface mount assembly technique (SMT) using a high precision placement apparatus can be applied because the photodetectors can be directly attached on a PLC by die bonding or flip chip bonding process. Moreover, an electrical connection path between the photodetectors and signal processing devices can also be very short.
However, in order to implement the above described vertical optical coupling scheme, light path transforming apparatus which perpendicularly changes a light path with respect to a substrate is required inside the optical waveguide.
An optical coupling scheme between optical waveguides and photodetectors using the above-described light path transforming apparatus is disclosed in the U.S. Pat. No. 4,904,036 filed by AT&T Co. and Bell Laboratories. As a recent example, it is suggested in a paper titled “Photonic integrated circuits based on silica and polymer PLC” written by T. Izuhara, et al. and published in SPIE proceeding Vol. 8628 862807-1 (2013).
FIG. 1 is a view illustrating a cross-sectional surface of an optical integrated circuit as prior art in which a light path turning mirror is embedded inside an optical waveguide and a photodetector is attached on the upper side of the light path turning mirror in order to describe a principle of vertical optical coupling scheme.
Referring to FIG. 1, an optical integrated circuit is provided with an optical waveguide consisted of a lower cladding layer 21, a core layer 22, and an upper cladding layer 23, and a groove 30 formed crossing the optical waveguide perpendicularly on a substrate 10.
Wherein, a sidewall 31 of the groove 30 crossing the optical waveguide through which light enters or exits is substantially perpendicular to a substrate, and an another sidewall 32 at the opposite side of the sidewall 31 is inclined about 45° with respect to the substrate 10 and is mirror-like polished for good light reflection.
In the above-described optical integrated circuit, light propagated through the core layer 22 substantially parallel to the substrate 10 exits to air from the sidewall 31 of the groove 30. The light which exited the optical waveguide and propagated through the air, is reflected by an inclined mirror surface 32, and turns its path upwardly and incidents into an active region of a photodetector 50 bonded by metal bonding pads 40 and solder bumps above the mirror surface 32, as shown by a reference numeral 35.
Wherein, when a light emitting VCSEL is applied instead of the photodetector 50, the direction of the light path 35 is opposite thereto.
By the way, in the above-described structure, when the optical waveguide is made of polymer as disclosed in the U.S. Pat. No. 7,995,875 or made of silicon as disclosed in the US Patent Publication No. 2014/0205234, the above-described mirror surface 32 can be fabricated relatively easily inside the optical waveguide. However, when the optical waveguide material is silica, it is very difficult to fabricate the above-described mirror surface 32 inside the optical waveguide because silica is known as a material very difficult to machine using dry etching techniques.
Conventionally, an anisotropic dry etching technique generally used for the manufacturing of optical waveguide is also used for the fabrication of light path turning mirror. In general, anisotropic etching forms grooves having a vertical sidewall. In contrast to the conventional grooves that could be formed by normal anisotropic dry etching techniques, a groove for vertical coupling requires that the sidewall 31 through which light enters or exits to or from the waveguide has to substantially perpendicular to the substrate 10 and the mirror surface 32 facing the vertical sidewall 31 has to have about 45° slope with respect to the substrate 10. Such a particular features of the groove (30) used for the exemplified applications (U.S. Pat. No. 7,995,875 and 2014/0205234) make difficult to implement such grooves by using the conventional anisotropic dry etching techniques and even more with a single processing step. In more detail, etching a groove having two different sidewall slopes as described above is very difficult. In despite of such difficulty, several methods of manufacturing the above-described light path turning mirror inside the optical waveguide have been disclosed. U.S. Pat. No. 8,236,481 disclosed by Google is one of the examples. However, most of the manufacturing methods proposed so far have several drawbacks; it requires very complex and high cost manufacturing processes such as electron-beam lithography, and in addition to that, the quality of the mirror surface and reproducibility of the slope angle are unsatisfactory.